Exhaust emission control device

ABSTRACT

An exhaust emission control device includes an exhaust pipe, a catalyst in the pipe promoting reduction reaction of nitrogen oxide in exhaust air, a supply device for supplying an additive agent to an upstream side of the catalyst, a temperature detecting device for detecting exhaust air temperature, and a control device for controlling the supply device to regulate the additive agent amount supplied into the pipe. The control device stops supply of the additive agent when exhaust air temperature is lower than generation temperature needed to generate the reducing agent from the additive agent, and supplies the additive agent when exhaust air temperature is equal to or higher than generation temperature. When a predetermined time has elapsed after a start of supply of the additive agent, the control device reduces the additive agent amount supplied, compared to the additive agent amount supplied before the predetermined time has elapsed.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by referenceJapanese Patent Application No. 2007-301757 filed on Nov. 21, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exhaust emission control device forreducing nitrogen oxide in exhaust air of an internal combustion enginesuch as a diesel engine. The invention is effectively applied to avehicle.

2. Description of Related Art

According to an exhaust emission control device for reducing a nitrogenoxide (NOx) included in exhaust air of an internal combustion enginesuch as a diesel engine, the nitrogen oxide is purified (reduced) byproviding in an exhaust pipe a catalyst that promotes a reductionreaction and by injecting an additive agent such as a urea watersolution into exhaust air flowing into the catalyst (see, for example,JP2003-293739A).

More specifically, Urea (CO(NH2)2) injected into exhaust air ishydrolyzed by exhaust heat (CO(NH2)2+H2O→2NH3+CO2) to generate ammonia(NH3), which is a reducing agent. Then, the nitrogen oxide is reduced byreaction between the nitrogen oxide and the ammonia through thecatalyst.

Exhaust-gas temperature needs to be equal to or higher than atemperature of 170° C. to 175° C. (hereinafter referred to as ammoniageneration temperature) in order to hydrolyze urea by exhaust heat.Therefore, even if urea is added to the exhaust air when the exhaust-gastemperature is lower than the ammonia generation temperature, thenitrogen oxide cannot be reduced by the ammonia, and moreover, the ureais released into the atmosphere without being hydrolyzed, so that theurea is wastefully consumed.

SUMMARY OF THE INVENTION

The present invention addresses the above disadvantages. Thus, it is anobjective of the present invention to prevent an additive agent, such asurea, from being consumed wastefully, and to reliably reduce (purify)nitrogen oxide.

To achieve the objective of the present invention, there is provided anexhaust emission control device for generating a reducing agent from anadditive agent by use of heat of exhaust air discharged from an internalcombustion engine and for reducing nitrogen oxide included in theexhaust air by the reducing agent. The device includes an exhaust pipe,a catalyst, a supply means, a temperature detecting means, and a controlmeans. The exhaust pipe defines a passage for the exhaust air. Thecatalyst is disposed in the exhaust pipe. The catalyst is capable ofpromoting reduction reaction of the nitrogen oxide in the exhaust air.The supply means is for supplying the additive agent to an upstream sideof the catalyst in a flow direction of the exhaust air. The temperaturedetecting means is for detecting temperature of the exhaust air flowingthrough the exhaust pipe. The control means is for controlling thesupply means so as to regulate an amount of the additive agent suppliedinto the exhaust pipe. The control means stops the supply of theadditive agent when the temperature of the exhaust air detected by thetemperature detecting means is lower than a generation temperature thatis needed to generate the reducing agent from the additive agent, andsupplies the additive agent when the temperature of the exhaust air isequal to or higher than the generation temperature. When a predeterminedtime has elapsed after a start of the supply of the additive agent, thecontrol means reduces the amount of the additive agent supplied, ascompared to the amount of the additive agent supplied before thepredetermined time has elapsed. The “predetermined time” may be a “timethat is equal to or shorter than a time in which an amount of thereducing agent generated exceeds an amount that is able to be adsorbedto the catalyst 3”.

To achieve the objective of the present invention, there is alsoprovided an exhaust emission control device for generating a reducingagent from an additive agent by use of heat of exhaust air dischargedfrom an internal combustion engine and for reducing nitrogen oxideincluded in the exhaust air by the reducing agent. The device includesan exhaust pipe, a catalyst, a supply means, a temperature detectingmeans, a reducing agent detecting means, and a control means. Theexhaust pipe defines a passage for the exhaust air. The catalyst isdisposed in the exhaust pipe. The catalyst is capable of promotingreduction reaction of the nitrogen oxide in the exhaust air. The supplymeans is for supplying the additive agent to an upstream side of thecatalyst in a flow direction of the exhaust air. The temperaturedetecting means is for detecting temperature of the exhaust air flowingthrough the exhaust pipe. The reducing agent detecting means is disposedon a downstream side of the catalyst in the flow direction of theexhaust air for detecting the reducing agent. The control means is forcontrolling the supply means so as to regulate an amount of the additiveagent supplied into the exhaust pipe. The control means stops the supplyof the additive agent when the temperature of the exhaust air detectedby the temperature detecting means is lower than a generationtemperature that is needed to generate the reducing agent from theadditive agent, and supplies the additive agent when the temperature ofthe exhaust air is equal to or higher than the generation temperature.When the reducing agent that is equal to or larger than a predeterminedvalue is detected by the reducing agent detecting means after a start ofthe supply of the additive agent, the control means reduces the amountof the additive agent supplied, as compared to the amount of theadditive agent supplied before the reducing agent that is equal to orlarger than the predetermined value is detected.

Furthermore, to achieve the objective of the present invention, there isprovided an exhaust emission control device for generating a reducingagent from an additive agent by use of heat of exhaust air dischargedfrom an internal combustion engine and for reducing nitrogen oxideincluded in the exhaust air by the reducing agent. The device includesan exhaust pipe, a catalyst, a supply means, a temperature detectingmeans, and a control means. The exhaust pipe defines a passage for theexhaust air. The catalyst is disposed in the exhaust pipe. The catalystis capable of promoting reduction reaction of the nitrogen oxide in theexhaust air. The supply means is for supplying the additive agent to anupstream side of the catalyst in a flow direction of the exhaust air.The temperature detecting means is for detecting temperature of theexhaust air flowing through the exhaust pipe. The control means is forcontrolling the supply means so as to regulate an amount of the additiveagent supplied into the exhaust pipe. The control means stops the supplyof the additive agent when the temperature of the exhaust air detectedby the temperature detecting means is lower than a generationtemperature that is needed to generate the reducing agent from theadditive agent, and supplies the additive agent when the temperature ofthe exhaust air is equal to or higher than the generation temperature.When the temperature of the exhaust air is equal to or higher than apredetermined temperature that is higher than the generationtemperature, the control means reduces the amount of the additive agentsupplied, as compared to the amount of the additive agent supplied whenthe temperature of the exhaust air is equal to or higher than thegeneration temperature and is lower than the predetermined temperature.

In addition, to achieve the objective of the present invention, there isprovided an exhaust emission control device for generating a reducingagent from an additive agent by use of heat of exhaust air dischargedfrom an internal combustion engine and for reducing nitrogen oxideincluded in the exhaust air by the reducing agent. The device includesan exhaust pipe, a catalyst, a supply means, a temperature detectingmeans, and a control means. The exhaust pipe defines a passage for theexhaust air. The catalyst is disposed in the exhaust pipe. The catalystis capable of promoting reduction reaction of the nitrogen oxide in theexhaust air. The supply means is for supplying the additive agent to anupstream side of the catalyst in a flow direction of the exhaust air.The temperature detecting means is for detecting temperature of theexhaust air flowing through the exhaust pipe. The control means is forcontrolling the supply means so as to regulate an amount of the additiveagent supplied into the exhaust pipe. The control means stops the supplyof the additive agent when the temperature of the exhaust air detectedby the temperature detecting means is lower than a generationtemperature that is needed to generate the reducing agent from theadditive agent, and supplies the additive agent when the temperature ofthe exhaust air is equal to or higher than the generation temperature.When at least one of the following conditions is satisfied, the controlmeans reduces the amount of the additive agent supplied, as compared tothe amount of the additive agent supplied before the one of thefollowing conditions is satisfied: the temperature of the exhaust air isequal to or higher than a predetermined temperature that is higher thanthe generation temperature; and a predetermined time has elapsed after astart of the supply of the additive agent.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with additional objectives, features andadvantages thereof, will be best understood from the followingdescription, the appended claims and the accompanying drawings in which:

FIG. 1 is a schematic diagram illustrating an exhaust emission controldevice according to a first embodiment of the invention;

FIG. 2 is a flow chart illustrating characteristic workings of theexhaust emission control device according to the first embodiment;

FIG. 3 is a graph illustrating a relationship among an injection amountof an additive agent (urea), an exhaust-gas temperature, and timeaccording to the first embodiment;

FIG. 4 is a diagram illustrating a relationship between the injectionamount of the additive agent (urea) and the exhaust-gas temperatureaccording to the first embodiment;

FIG. 5 is a schematic diagram illustrating an exhaust emission controldevice according to a second embodiment of the invention; and

FIG. 6 is a flow chart illustrating characteristic workings of theexhaust emission control device according to the second embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments are applications of an exhaust emission control device ofthe invention to a urea SCR (Selective Catalytic Reduction) system of adiesel engine for vehicles. The embodiments are described below withreference to the drawings.

First Embodiment Configuration of an Exhaust Emission Control Device

As shown in FIG. 1, an exhaust pipe 2 defines a passage for exhaust airdischarged from a diesel internal combustion engine 1. An SCR catalyst 3(hereinafter referred to as catalyst 3), which promotes reductionreaction of nitrogen oxide in exhaust air, and a DPF (Diesel ParticulateFilter) 4 for capturing particulate matter such as soot contained inexhaust air are provided in the exhaust pipe 2. The DPF 4 is located onan upstream side (engine side) of the catalyst 3 in an exhaust flowdirection.

A supply valve 5 is a supply means for supplying an additive agent (ureawater solution in a first embodiment) used for reduction reaction intothe exhaust pipe 2 on an upstream side of the catalyst 3 in a flowdirection of the exhaust air. An additive-agent tank 6 is a tank meansfor storing the additive agent supplied to the exhaust pipe 2.

An additive-agent pump 7 is a pump means for pumping the additive agentstored in the additive-agent tank 6 to the supply valve 5. An exhausttemperature sensor 8 is a temperature detecting means for detecting thetemperature of exhaust air discharged from the internal combustionengine 1 A NOx sensor 9 is a NOx detecting means for detecting thenitrogen oxide included in exhaust air which has passed through thecatalyst 3.

In the first embodiment, the exhaust-air temperature is detected nearthe inlet of the catalyst 3, and the nitrogen oxide is detected near theoutlet of the catalyst 3. Detection signals of the exhaust temperaturesensor 8 and the NOx sensor 9 are inputted into an electronic controlunit (hereinafter referred to as ECU) 10. The ECU 10 controls workingsof the supply valve 5 and the additive-agent pump 7 based on thesedetection signals of the exhaust temperature sensor 8 and the NOx sensor9 and the like.

The ECU 10 is a widely-known microcomputer including a centralprocessing unit (CPU) 10A, a random access memory (RAM) 10B, a read-onlymemory (ROM) 10C, and a timer 10D, which keeps time. A program forcontrolling the supply valve 5 and the like is stored in the ROM 10C.

(Basic Workings of the Exhaust Emission Control Device)

The exhaust emission control device hydrolyzes (CO(NH2)2+H2O→2NH3+CO2)urea (CO(NH2)2), which is the additive agent injected into exhaust air,using exhaust heat so as to generate ammonia (NH3), which is a reducingagent. Then, the exhaust emission control device causes reaction betweenthe nitrogen oxide and the ammonia through the catalyst 3 so as topurify (reduce) the nitrogen oxide.

Meanwhile, when exhaust-gas temperature is equal to or larger than atemperature of 170° C. to 175° C. (hereinafter referred to as ammoniageneration temperature), urea is hydrolyzed to generate ammonia.However, when the exhaust-gas temperature is lower than the ammoniageneration temperature, urea is released into the atmosphere withoutbeing hydrolyzed, and accordingly urea is consumed wastefully.

(Characteristic Workings of the Exhaust Emission Control Device)

As shown in FIG. 2, the exhaust emission control device (the supplyvalve 5 and the additive-agent pump 7) is started at the same time asstarting of the internal combustion engine 1. The amount of the additiveagent supplied is controlled (hereinafter referred to as normal control)normally, based on the temperature of exhaust air (detection temperatureof the exhaust temperature sensor 8) discharged from the internalcombustion engine 1, the amount of the nitrogen oxide (detection valueof the NOx sensor 9) contained in the exhaust air, and the like.

The control (hereinafter referred to as reducing agent slip inhibitorycontrol) shown in FIG. 2 is started at the same time as the normalcontrol and performed independently of the normal control. The reducingagent slip inhibitory control is performed as follows.

That is, when the exhaust-gas temperature which the exhaust temperaturesensor 8 has detected is smaller than the ammonia generationtemperature, supply of the additive agent (urea) to the exhaust pipe 2is stopped. On the other hand, when the exhaust-gas temperature is equalto or larger than the ammonia generation temperature, the additive agentis supplied to the exhaust pipe 2.

An amount of the additive agent supplied into the exhaust pipe 2 is suchthat a larger amount of the reducing agent than an amount of thereducing agent needed to reduce all the nitrogen oxide contained in theexhaust air discharged from the internal combustion engine 1 isgenerated.

Meanwhile, the reducing agent generated through the hydrolysis isadsorbed to the catalyst 3, and then produces reduction reaction withnitrogen oxide through the catalyst 3. An excessively generated reducingagent is kept adsorbed to the catalyst 3, and when the exhaust-gastemperature lowers to less than the ammonia generation temperature andthereby a supply of the additive agent stops, the reducing agent, whichis excessively generated and adsorbed to the catalyst 3, is consumed onthe reduction reaction and accordingly the nitrogen oxide is purified.

However, if the exhaust-gas temperature continues being equal to orlarger than the ammonia generation temperature over a long time, a totalamount of the generated reducing agent exceeds a threshold limit of theadsorption by the catalyst 3, and accordingly, the generated reducingagent is released without being used for the reduction reaction.

In the first embodiment, on at least one of a condition that theexhaust-gas temperature is equal to or higher than a predeterminedtemperature (hereinafter referred to as supply stop temperature) that ishigher than the ammonia generation temperature, and a condition that apredetermined time (hereinafter referred to as a supply stop time) haselapsed since a start of supply of the additive agent, the supply amountof the additive agent is reduced compared to an amount of the additiveagent that has been supplied into the exhaust pipe 2 before one of thetwo conditions is satisfied.

“Before one of the two conditions is satisfied” is “when the exhaust-gastemperature is equal to or higher than the ammonia generationtemperature and lower than the supply stop temperature” or “before thesupply stop time elapses after the exhaust-gas temperature becomes equalto or higher than the ammonia generation temperature”. Therefore, the“amount of the additive agent that has been supplied into the exhaustpipe 2 before one of the two conditions is satisfied” is such that, asdescribed above, a larger amount of the reducing agent than an amount ofthe reducing agent needed to reduce all the nitrogen oxide contained inthe exhaust air discharged from the internal combustion engine 1 isgenerated.

A supply amount of the additive agent, which can generate an amount ofthe reducing agent needed to reduce all the nitrogen oxide contained inthe exhaust air is hereinafter referred to as a normal amount, and asupply amount of the additive agent supplied before one of the twoconditions is satisfied is hereinafter referred to as more than thenormal amount.

Details of the above-described workings are explained below withreference to a flow chart in FIG. 2. When the reducing agent slipinhibitory control is started, whether the detection temperature(hereinafter referred to as exhaust-gas temperature) of the exhausttemperature sensor 8 is lower than the ammonia generation temperature T1is determined (S1).

If it is determined that the exhaust-gas temperature is lower than theammonia generation temperature T1 (S1: YES), the time keeping by thetimer 10D is started or continued (S7) after the supply (injection) ofthe reducing agent is stopped and time that is kept by the timer 10D isinitialized (S2).

If it is determined that the exhaust-gas temperature is not lower thanthe ammonia generation temperature T1, i.e., the exhaust-gas temperatureis equal to or higher than the ammonia generation temperature T1(S1:NO), whether the time kept by the timer 10D is equal to or longerthan the supply stop time is determined (S3). If it is determined thatthe kept time is equal to or longer than the supply stop time (S3: YES),the time keeping by the timer 10D is started or continued (S7) after thenormal amount of the additive agent is supplied into the exhaust pipe 2(S5).

If it is determined that the kept time is not equal to or longer thanthe supply stop time (S3: NO), it is determined whether the exhaust-gastemperature is equal to or higher than the supply stop temperature T2(S4). If it is determined that the exhaust-gas temperature is equal toor higher than the supply stop temperature T2 (S4: YES), the timekeeping by the timer 10D is started or continued (S7) after the normalamount of the additive agent is supplied into the exhaust pipe 2 (S5).

If it is determined that the exhaust-gas temperature is not equal to orhigher than the supply stop temperature T2 (S4: NO), the time keeping bythe timer 10D is started or continued (S7) after the additive agent morethan the normal amount is supplied into the exhaust pipe 2 (S6). Then,after the processing S7 is performed, the processing S1 is performedagain.

(Characteristics of the Exhaust Emission Control Device)

In the first embodiment, if the exhaust-gas temperature is lower thanthe ammonia generation temperature T1, the supply of the additive agentis stopped as shown in FIG. 3 and FIG. 4. Accordingly, the additiveagent is prevented from being consumed wastefully.

If the exhaust-gas temperature is equal to or higher than the ammoniageneration temperature T1, the additive agent more than the normalamount is supplied. Accordingly, the nitrogen oxide is reduced(purified) by the reducing agent generated from the additive agent, anda part of the generated reducing agent is adsorbed to the catalyst 3. Ifthe exhaust-gas temperature is lower than the ammonia generationtemperature T1 and thereby the supply of the additive agent stops, thenitrogen oxide is reduced by the reducing agent adsorbed to the catalyst3.

If the exhaust-gas temperature is equal to or higher than the ammoniageneration temperature T1, as described above, a part of the generatedreducing agent is adsorbed to the catalyst 3 without being used for thereductive reaction, and then used for the reductive reaction when theexhaust-gas temperature is lower than the ammonia generation temperatureT1. However, when the amount of the generated reducing agent exceeds anamount that is able to be adsorbed to the catalyst 3, the generatedreducing agent is discharged without being used for the reduction of thenitrogen oxide. Accordingly, the additive agent is consumed wastefully.

In the first embodiment, as shown in FIG. 3, if the supply stop time haselapsed after the start of supply of the additive agent, the supplyamount of the additive agent is reduced compared to a supply amountbefore the predetermined time has elapsed. Accordingly, the amount ofthe reducing agent generated is prevented from exceeding the amount thatis able to be adsorbed to the catalyst 3.

Moreover, when the reducing agent continues being generated from theadditive agent, the exhaust-gas temperature gradually increases overtime. Accordingly, by reducing the supply amount of the additive agentwhen the exhaust-gas temperature becomes equal to or higher than thesupply stop temperature T2, the amount of the generated reducing agentis prevented from exceeding the amount that is able to be adsorbed tothe catalyst 3.

In addition, as is obvious from the above description, the supply stoptemperature T2 is a temperature corresponding to, for example, anexhaust-gas temperature at the time the supply stop time elapses afterthe ammonia generation temperature T1. The supply stop time and thesupply stop temperature T2 are determined appropriately based onspecifications of the catalyst 3 or specifications of the internalcombustion engine 1.

As described above, in the first embodiment, the additive agent isprevented from being consumed wastefully, and the nitrogen oxide isreliably reduced (purified). In the first embodiment, the supply valve 5corresponds to the “supply means”, the exhaust temperature sensor 8corresponds to the “temperature detecting means”, and the ECU 10corresponds to a “control means”.

Second Embodiment General Description of an Exhaust Emission ControlDevice According to a Second Embodiment

In the second embodiment, as shown in FIG. 5, an ammonia sensor 11 fordetecting ammonia, which is a reducing agent, is disposed on adownstream side of a catalyst 3 in a flow direction of the exhaust air.A larger amount of an additive agent than a normal amount is supplieduntil the ammonia sensor 11 detects ammonia of a predetermined value orabove after an exhaust-gas temperature becomes equal to or higher thanan ammonia generation temperature T1. The normal amount of the additiveagent is supplied until the exhaust-gas temperature becomes lower thanthe ammonia generation temperature T1 after the ammonia sensor 11detects ammonia.

The above-described workings are explained in detail with reference toFIG. 6.

(Characteristic Workings of the Exhaust Emission Control Device of theSecond Embodiment)

When reducing agent slip inhibitory control is started, it is firstdetermined whether the exhaust-gas temperature is lower than the ammoniageneration temperature T1 (S11).

If it is determined that the exhaust-gas temperature is lower than theammonia generation temperature T1 (S11: YES), a supply (injection) ofthe reducing agent is stopped and time that is kept by a timer 10D isinitialized (S12). Then, when a predetermined time has elapsed after thetime keeping by the timer 10D is started or continued (S16), theprocessing S11 is performed again.

On the other hand, if it is determined that the exhaust-gas temperatureis not lower than the ammonia generation temperature T1, i.e., theexhaust-gas temperature is equal to or higher than the ammoniageneration temperature T1 (S11: NO), whether the ammonia sensor 11detects ammonia of the predetermined value or above is determined (S13).If it is determined that ammonia of the predetermined value or above isdetected (S13: YES), the normal amount of the additive agent is suppliedinto the exhaust pipe 2 (S14). Then, the processing S11 is performedagain when the predetermined time has elapsed (S16).

If it is determined that ammonia of the predetermined value or above isnot detected (S13: NO), the additive agent more than the normal amountis supplied into the exhaust pipe 2 (S15). Then, the processing S11 isperformed again when the predetermined time has elapsed (S16).

(Characteristics of the Exhaust Emission Control Device of the SecondEmbodiment)

In the second embodiment as well, similar to the above-describedembodiment, if the exhaust-gas temperature is lower than the ammoniageneration temperature T1, the supply of the additive agent is stoppedand the nitrogen oxide is reduced by the reducing agent adsorbed to thecatalyst 3. Therefore, the additive agent is prevented from beingconsumed wastefully.

If the exhaust-gas temperature is equal to or higher than the ammoniageneration temperature T1, the additive agent is supplied and thenitrogen oxide is reduced (purified) by the reducing agent generatedfrom the additive agent. Furthermore, a part of the generated reducingagent is adsorbed to the catalyst 3.

When ammonia (reducing agent) of the predetermined value or above isdetected by the ammonia sensor 11 after the supply of the additive agentis started, the supply amount of the additive agent is reduced ascompared to the supply amount before the reducing agent has beendetected. Thus, the amount of the generated reducing agent is preventedfrom exceeding the amount that is able to be adsorbed to the catalyst 3.

Therefore, in the second embodiment as well, the additive agent isprevented from being consumed wastefully, and the nitrogen oxide isreliably reduced (purified). In the second embodiment, the ammoniasensor 11 corresponds to a “reducing agent detecting means”.

Other Embodiments

In the above embodiments, urea is used as an additive agent However, theinvention is not limited to the above. That is, a reducing agent otherthan ammonia, or an additive agent that generates this reducing agentmay be used.

According to the invention, the first embodiment and the secondembodiment may be combined.

Additional advantages and modifications will readily occur to thoseskilled in the art. The invention in its broader terms is therefore notlimited to the specific details, representative apparatus, andillustrative examples shown and described.

1. An exhaust emission control device for generating a reducing agentfrom an additive agent by use of heat of exhaust air discharged from aninternal combustion engine and for reducing nitrogen oxide included inthe exhaust air by the reducing agent, the device comprising: an exhaustpipe defining a passage for the exhaust air; a catalyst disposed in theexhaust pipe, the catalyst being capable of promoting reduction reactionof the nitrogen oxide in the exhaust air; a supply means for supplyingthe additive agent to an upstream side of the catalyst in a flowdirection of the exhaust air; a temperature detecting means fordetecting temperature of the exhaust air flowing through the exhaustpipe; and a control means for controlling the supply means so as toregulate an amount of the additive agent supplied into the exhaust pipe,wherein: the control means stops the supply of the additive agent whenthe temperature of the exhaust air detected by the temperature detectingmeans is lower than a generation temperature that is needed to generatethe reducing agent from the additive agent, and supplies the additiveagent when the temperature of the exhaust air is equal to or higher thanthe generation temperature; and when a predetermined time has elapsedafter a start of the supply of the additive agent, the control meansreduces the amount of the additive agent supplied, as compared to theamount of the additive agent supplied before the predetermined time haselapsed.
 2. An exhaust emission control device for generating a reducingagent from an additive agent by use of heat of exhaust air dischargedfrom an internal combustion engine and for reducing nitrogen oxideincluded in the exhaust air by the reducing agent, the devicecomprising: an exhaust pipe defining a passage for the exhaust air; acatalyst disposed in the exhaust pipe, the catalyst being capable ofpromoting reduction reaction of the nitrogen oxide in the exhaust air; asupply means for supplying the additive agent to an upstream side of thecatalyst in a flow direction of the exhaust air; a temperature detectingmeans for detecting temperature of the exhaust air flowing through theexhaust pipe; a reducing agent detecting means disposed on a downstreamside of the catalyst in the flow direction of the exhaust air fordetecting the reducing agent; and a control means for controlling thesupply means so as to regulate an amount of the additive agent suppliedinto the exhaust pipe, wherein: the control means stops the supply ofthe additive agent when the temperature of the exhaust air detected bythe temperature detecting means is lower than a generation temperaturethat is needed to generate the reducing agent from the additive agent,and supplies the additive agent when the temperature of the exhaust airis equal to or higher than the generation temperature; and when thereducing agent that is equal to or larger than a predetermined value isdetected by the reducing agent detecting means after a start of thesupply of the additive agent, the control means reduces the amount ofthe additive agent supplied, as compared to the amount of the additiveagent supplied before the reducing agent that is equal to or larger thanthe predetermined value is detected.
 3. An exhaust emission controldevice for generating a reducing agent from an additive agent by use ofheat of exhaust air discharged from an internal combustion engine andfor reducing nitrogen oxide included in the exhaust air by the reducingagent, the device comprising: an exhaust pipe defining a passage for theexhaust air; a catalyst disposed in the exhaust pipe, the catalyst beingcapable of promoting reduction reaction of the nitrogen oxide in theexhaust air; a supply means for supplying the additive agent to anupstream side of the catalyst in a flow direction of the exhaust air; atemperature detecting means for detecting temperature of the exhaust airflowing through the exhaust pipe; and a control means for controllingthe supply means so as to regulate an amount of the additive agentsupplied into the exhaust pipe, wherein: the control means stops thesupply of the additive agent when the temperature of the exhaust airdetected by the temperature detecting means is lower than a generationtemperature that is needed to generate the reducing agent from theadditive agent, and supplies the additive agent when the temperature ofthe exhaust air is equal to or higher than the generation temperature;and when the temperature of the exhaust air is equal to or higher than apredetermined temperature that is higher than the generationtemperature, the control means reduces the amount of the additive agentsupplied, as compared to the amount of the additive agent supplied whenthe temperature of the exhaust air is equal to or higher than thegeneration temperature and is lower than the predetermined temperature.4. An exhaust emission control device for generating a reducing agentfrom an additive agent by use of heat of exhaust air discharged from aninternal combustion engine and for reducing nitrogen oxide included inthe exhaust air by the reducing agent, the device comprising: an exhaustpipe defining a passage for the exhaust air; a catalyst disposed in theexhaust pipe, the catalyst being capable of promoting reduction reactionof the nitrogen oxide in the exhaust air; a supply means for supplyingthe additive agent to an upstream side of the catalyst in a flowdirection of the exhaust air; a temperature detecting means fordetecting temperature of the exhaust air flowing through the exhaustpipe; and a control means for controlling the supply means so as toregulate an amount of the additive agent supplied into the exhaust pipe,wherein: the control means stops the supply of the additive agent whenthe temperature of the exhaust air detected by the temperature detectingmeans is lower than a generation temperature that is needed to generatethe reducing agent from the additive agent, and supplies the additiveagent when the temperature of the exhaust air is equal to or higher thanthe generation temperature; and when at least one of the followingconditions is satisfied, the control means reduces the amount of theadditive agent supplied, as compared to the amount of the additive agentsupplied before the one of the following conditions is satisfied: thetemperature of the exhaust air is equal to or higher than apredetermined temperature that is higher than the generationtemperature; and a predetermined time has elapsed after a start of thesupply of the additive agent.